Under the proper conditions, two moving gratings that are superimposed are perceived as a plaid moving in the vector average of the 2 grating directions. Much research has been done both psychophysically and through neuronal recordings to understand the process of integrating the two gratings into a single plaid object. A field of dots (random dot kinetograms) that moves coherently is perceived as a surface. When two dot fields are superimposed upon each other, moving in different directions, no such integration occurs. Instead, the dots are segmented into two distinct objects.

We investigated whether the perceived directions of motion of two superimposed surfaces would still be affected by the process of direction integration. Subjects fixated a central cross while an aperture containing 2 surfaces moving in different directions appeared in the lower right or lower left visual fields. After 1000 ms, the surfaces and fixation cross were removed while a white circular outline of the aperture appeared. Subjects used a mouse to click on the perceived directions of motion for each of the 2 surfaces. We expected to find that the difference in the perceived directions would be less than the actual difference between the directions, as this would be consistent with (weak) integration. Surprisingly, we found the opposite effect. The difference in perceived directions was significantly larger than the difference in actual directions.

These results suggest that unlike the integration of moving gratings into a plaid, superimposed surfaces comprised of random dot kinetograms are repulsed. The key factor is that the RDKs are automatically segmented into two objects providing a substrate for competitive interactions. Thus, the repulsion of perceived direction is likely due to competitive circuits previously identified for attentional modulation in area MT.